Caustic recovery from green liquor of agro-based paper mills using electrolysis

Highlights

• Caustic recovery from green liquor using electrolysis.

• Feed concentration, current density, flow rate, cell geometry etc. were optimized.

• Current efficiency/energy consumption were evaluated with optimized process conditions.

• Results were compared between the synthetic and industrial green liquor.

Abstract

In the chemical recovery section of pulp and paper mills, the cooking liquor (white liquor, mainly caustic, NaOH) is regenerated by addition of fresh lime (CaO) in green liquor solution (rich in sodium carbonate, Na₂CO₃), where calcium carbonate (CaCO₃) get precipitated leaving NaOH in solution (causticizing process). Subsequently, CaO is regenerated from precipitated CaCO₃ following washing/drying/calcination steps which are highly energy intensive and environmentally unsafe. The current investigation reports conversion of green liquor to caustic solution using electrolysis through a cation exchange membrane (CEM) placed between two electrodes. Process parameters e.g. current density, feed concentration, flow velocity, flow promoter, electrode-membrane spacing, role of gravity flow were systematically investigated to find optimum values. Process optimization was performed with Na₂CO₃ (highest constituent of green liquor) solution to avoid complexity, which were then extended to complex salt mixtures of green liquors (synthetic/ industrial sample). Performance analysis of any batch was carried out based on moles of NaOH produced and associated current efficiency and energy consumption observed. With 600 A·m⁻² current density, 1.93 mol·L⁻¹ of NaOH could be produced in 6 h of electrolysis through a cell of 66 × 10⁻⁴ m² of CEM, where flow rates of both feed and catholyte streams were counter current and fixed at 3.472 × 10⁻³ m·s⁻¹.

Introduction

Pulp and paper mills fall in the category of highly energy intensive industries consuming huge amount of natural resources in the form of wood, freshwater and fossil fuels per ton of paper produced [1]. Besides this, the air, water and solid wastes released/discharged from these mills are highly hazardous in nature and hence are serious concerns for the environment [2]. In these mills waste streams are being produced almost in each stage due to extensive washing of digested pulp etc. producing large volume of waste water. Therefore, advanced technologies are in great demand in these mills.

The conventional pulping of wood chips (recovery of pulp from wood) are carried out using two methods i.e. (i) Kraft pulping and (ii) Mechanical pulping and both of these processes use caustic, NaOH solution for separation of lignin from cellulosic fibers, known as digestion. This is done at high temperature (<480 °C) and high pressure (90–110 bar) [3]. Sodium sulfide (Na₂S) is added only in case of Kraft process, which helps in easy delignification of wood fibers without much affecting the cellulose fiber length thus, the quality and yield of fibers improve. After digestion of wood, the fibrous pulp is screened out from the lignin solution called black liquor (polyhydroxy phenolic compounds and inorganic compounds) [4], [5], [6].

Black liquor consists of 65–85% of solid and 10–50% of the mass is contributed by lignin [7] and fiber residues, thus black liquor is of high Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) [5]. The lignin mass, rich is aromatic polyhydroxy phenolic compounds is a huge source of thermal energy, hence is combusted after drying using mostly Tomlinson recovery boiler and high pressure steam can be produced. At present almost 2 × 10⁸ ton of solidified black liquor produced globally are yielding ~2.4 × 10¹⁵ J of energy annually and the quantity is still growing with improved technologies [8]. The molten smelt obtained from recovery furnace is rich in Na₂CO₃ and Na₂S, which are mixed with water in a dissolution tank to obtain greenish solution (due to Na₂S) called green liquor. This solution is treated with lime, CaO (causticization) to regenerate a white liquor (NaOH and Na₂S), which can be recycled and the CaCO₃ precipitate is sent for washing/drying/calcination at very high temperature ~900 °C to regenerate CaO. The rotary lime-kiln, used for calcination of CaCO₃ is highly energy intensive, time consuming besides producing solid waste (lime mud), particulate matters. Additionally, presence of silica leads to clogging of kiln and shutdown, making the process time consuming and labor intensive.

Increasing awareness of environmental issues coupled with stringent pollution board norms are forcing industries to think of recycling, reducing or process modification of conventional steps. Good number of modern pulp mills are opting for recycling of waste papers and agricultural residues (wheat straw, rice straw, bagasse, etc.) as raw materials instead of conventional wood chips [9]. Mostly, small-scale pulp mills are using agro-residues to meet shortage of fresh wood [10], [11], [12].

Silica contents are slightly higher in agro-residues and mechanical or soda pulping (only NaOH) is effective for recovery of cellulose fibres. Higher silica content increases viscosity of black liquor thus, evaporator efficiency drops and energy cost increases [13], besides deposition of silica along evaporator tubes and chocking. Additionally, the recovery boiler, causticizing plant, and rotary kiln also encounter scaling issues [14], [15], hence small mills mostly dispose the smelt (furnace) at low cost to cement/detergent industries. Thus, fresh cooking liquor is to be charged for next batch of digestion. This can be overcome if NaOH can be regenerated through alternate route.

Electro-membrane processes are capable of selective separation of various ionic species and a right membrane can achieve desired recovery under applied potential without addition of any chemical. Electrolysis is one such technique, where ions are selectively separated through membrane placed between two electrodes. Under applied potential, ions get transported through membrane making the feed solution free of electrolyte, while enriching the adhering solution. Use of cation exchange membrane (CEM) leads to transport of metal ions (positively changed) and forming alkaline solution in the adhering compartment [16], while use of anion exchange membrane (AEM) results in corresponding acid stream [17], [18], [19]. During electrolysis, splitting of water molecules occur forming hydrogen and oxygen gases as follows:

Anode: H₂O (liq) → 1/2 O₂ (g) + 2H⁺ + 2e‾

Cathode: 2H⁺ + 2e‾ → H₂ (g)

Overall cell reaction: H₂O (liq) → H₂ (g) + 1/2 O₂ (g)

M/s Asahi Chemicals, Japan reported (1975) formation of chlorine (gas) and NaOH during electrolysis of NaCl through perfluorinated membranes [20]. NaOH production through electrolysis of sodium salts e.g. sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃) and NaCl using single CEM was investigated by Simon et al. [16], where the main objective was to investigate role of feed concentration and current density to achieve maximum NaOH production. Higher feed concentration increased Na⁺ transport resulting higher NaOH concentration. Influence of boundary layer thickness and concentration polarization along membrane were also showed strong influence in ion transport [21], [22], [23], [24], [25], [26], [27], where application of different promoters (twisted tape, rod, netted spacer, multilayer spacers and their arrangements etc.) were proposed. Even corrugation over membrane leading to boundary layer minimization were reported [28], [29].

Flow promoters placed between flat membranes promotes growth of eddies of different kinetic energies, which reduces boundary layer thickness and facilitate ion transport. Wide range of promoters (or spacers: rod, twisted tape, screw, rectangular winglet, etc.) were reported earlier [21], [22], [23], [24], [25], [27]. Isaacson et al. [26] reported 4 times improvement in ion transport with rod-type promoters. Chandra et al. [28] and Tedimeti et al. [29] reported minimization of shadow effect by applying corrugated channels. Netted spacer, twisted tape (TT), cylindrical rods [21], [27], multilayer spacer [30], and diamond shape spacer [31] are popular promoters. Hattenbach et al. [32] and Turek [33] reported role of membrane-electrode spacing and at constant velocity reduced spacing facilitates ion transport due to lowering of boundary layer thickness.

In this report, we investigate regeneration of NaOH from green liquor of agro-based pulping mills through electrolysis. Major constituents of agro-based green liquor are: Na₂CO₃ (~130 g·L⁻¹), NaOH (~38 g·L⁻¹), Na₂S (~5 g·L⁻¹) and silica. Process parameters (applied current density, flow rate, membrane-electrode spacing, feed concentration, spacer geometries, etc.) were optimized using only Na₂CO₃ solution, which were then extended for synthetic green liquor and industrial sample. Performances of any experiment were evaluated based on concentration of NaOH produced, current efficiency and energy consumption happened.